Silicon is ubiquitous in integrated circuits (ICs), microelectromechanical systems (MEMS), and sensing applications owing to the well-developed infrastructure for its manipulation into both electronic and structural elements. Typically, these applications are realized using standard lithographic procedures to generate transistors, lines, tiny gears, latches, sensors, etc. Current lithographic processes are often limited to a large extent by obtainable feature size and cost.
Furthermore, there is a growing interest in the use of organic molecules to control and tailor the interfacial characteristics of semiconductor surfaces to address emerging functional requirements for ICs, MEMS, and other micro- and nano-scale devices. In particular, self-assembled monolayers (SAMs) of organic molecules can provide a relatively ordered monolayer with a variety of chemical functional groups on a surface. The surface chemistry of organic molecules assembled on silicon surfaces has recently been reviewed. See Jillian M. Buriak, “Organometallic Chemistry on Silicon and Germanium Surfaces,” Chemical Reviews 102(5), 1371 (2002), which is incorporated herein by reference.
The Si—C bond is both thermodynamically and kinetically stable due to the high bond strength and low polarity of the bond. However, upon exposure to air, single-crystal silicon becomes rapidly coated with a native oxide that must be removed chemically with fluoride, or thermally under UHV conditions, to provide a Si—C bond-forming surface. Preferably, the surface of silicon can be prepared to provide a precursor surface that is stable enough to be handled at atmospheric pressure in the presence of solvent vapors and other contaminants, yet reactive enough for subsequent monolayer assembly. In particular, a hydride-terminated silicon surface has been found to be relatively stable in air for short periods. Several wet chemical and UHV approaches to self-assemble organic molecules can be carried out on such hydride-terminated surfaces. Wet chemical approaches to Si—C bond formation include hydrosilylation involving a radical initiator, thermally induced hydrosilylation, photochemical hydrosilylation, and electrochemical grafting.
Recent work has shown that diazonium molecules self assemble via an electron transfer mechanism on many conducting and semiconducting surfaces, such as silicon and metals. See Stewart et al., “Direct Covalent Grafting of Conjugated Molecules onto Si, GaAs, and Pd Surfaces from Aryldiazonium Salts,” J. Am. Chem. Soc. 126, 27 (2004), which is incorporated herein by reference. However, technologies are still required for the directed assembly of organic molecules onto silicon or other conductor or semiconductor surfaces. Such patterning is desirable to extend the utility of SAMs for specific applications, such as molecular electronic devices, molecular nanolithography, photovoltaic devices, and chemical and biological microsensors. In the related U.S. patent application Ser. No. 10/984,569 are described several methods for the directed assembly of organic molecules from the reaction of a diazonium salt precursor with a silicon surface. However, the diazonium assembly can be difficult to control, especially when using direct writing with negative bias.